[Qemu-devel] Debugging AIO hangs

[Daniel Dickinson]

Hi,

I  have reported Debian bug #474386 sometime ago, and in the process
tracked the hangs I was experiencing (consistently during windows
98se guest install, which is what I used to git bisect the commits, but
also at random times with different Ubuntu guests) to one of the
following three commits:

The last good revision is 2072 (git
d62ca2669be6d6af2c0cbda47abd7e51548060bf)

The first known bad commit is 2075 (git
83f640910acd7cd13ff8a603f29c46033c4fb00)

Which is where the aio code was introduced.  Since those commits
the hangs occur and have not disappeared with fairly recent SVN
builds.

My question is where I should start looking and what I should do to
start tracking down where in the code the hangs are occurring.  I
don't know aio and there is a lot of code to look at; I don't think
I'm going to figure it the problem purely by inspection and I have
limited time to work on this, so anything that makes my time spent
more useful would be appreciated.

I know the code has changed since those commits, but the problem
hasn't changed since then, so I am assuming there is a fairly basic
(yet hard to find) flaw that causes the aio to block, probably due
to deadlock, but possibly for other reasons.

FYI I have browsed the user forums and it looks like there are
other people who experience the same issues (random vm hangs in
various guests, and no luck with 98se), so I assume that it's not just
my hardware or specific to my system (but perhaps to debian-derived
systems as the other reports are from debian or ubuntu systems, IIRC).

Regards,

Daniel


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